The invention relates to a method for controlling pressurized screening devices in which fibre pulp is fed into the pressurized screening device and classified by means of a perforated screenplate into the accepted portion passing through the perforations of the screenplate and into the rejected portion which is removed from the screenplate. The perforated screenplate is treated by means of a device movable with respect thereto. The screenplate is cylindrical and the device for treating it rotates around the center axis of the screenplate.
Several different designs for pressurized screening devices of this kind are known, for instance, reference can be made to patent publications FI-56451 and FI-70059.
In screening, the purity of the accepted pulp and the capacity of the pressurized screening device are affected by the screenplate and by the device which moves relative to the screenplate and treats it. Perforation size in the screenplate has influence on both the capacity and the purity of the accepted portion. Also the surface profile, for example being profiled or smooth, has influence on the same quantities. The function of the device treating the screenplate, often called in technical language as the rotor, is to keep the screenplate clean, that is to break the fibre mat formations on its surface.
Experiments have shown that, in screening devices, the tangential velocity of the fibre pulp to be screened has a major effect on the capacity and the purity of the accepted fibre pulp. Experiments have revealed that, if the tangential velocity of the fibre pulp in the pressurized screening device approaches zero, the capacity of the pressurized screener will be high and the screening result will be poor. If, on the other hand, the tangential velocity of the fibre pulp in the pressurized screening device corresponds closely to that of the device (rotor) treating the screenplate, the capacity will become low and the screening result will become better.
The most common design of prior art is such that the fibre pulp in the screening device rotates much slower than the device (rotor) treating the screenplate. Situation becomes even worse in such pressurized screening devices where the tangential velocity of the fibre pulp varies in different parts of the screenplate, this effect becomes particularly crucial in the vertical direction of the screenplate in most designs of the pressurized screening devices presently in use. Therefore, the flow dynamics inside the pressurized screening device is unknown and indefinite. The situation in pressurized screening devices according to prior art is such that the tangential velocity of the fibre pulp is under control only in such pressurized screening devices where the pulp is forced to rotate along with the device (rotor) treating the screenplate.